Over the past several decades, the performance of light emitting diodes (LEDs) has increased in many ways, including improvements in efficiency, flux output, color rendering, stability of color, and stability of the correlated color temperature of white light. These improvements, and the high reliability of LEDs, make them useful for a wide range of high-brightness illumination applications, including automotive forward lighting and various display applications. However, in order for high-power LEDs to achieve the high lumens output and flux density (as a white light source) that is needed to replace conventional technologies such as incandescent bulbs, fluorescent lighting, and metal halide light sources, the LEDs must be driven at high current levels, which in turn results in high levels of heat generation. Special packaging techniques need to be employed to prevent the degradation of LED quantum efficiency, due to temperature increases. Although some LED applications allow the use of electrically-conducting submount or substrate materials, electrical insulation (high electrical resistance, or low electrical conductivity) is required in many applications. However, many of the best materials for good thermal conductivity are also electrically conducting. Therefore, the development of a high thermal conductivity LED submount or substrate material that also provides superior electrical insulation, is one of the key issues for addressing LED heat dissipation.
Several types of submount/substrate material, such as PCB (printed circuit board), MCPCB (metal core printed circuit board), ceramic substrate, direct copper bonded substrate and LTCC-M (low temperature co-fired ceramic on metal) substrate have been developed and employed in the prior art as LED submount packages.
The thermal conductivities of dielectric insulator materials such as PCB and MCPCB are about 0.36 W/m° K and about 2 W/m° K, respectively. For ceramic substrate and direct copper bonded substrate, the most frequently used dielectric insulator materials are Al2O3, and AlN. These materials have a higher thermal conductivity, with typical values of 20-230 W/m° K. For LTCC-M substrate, the major compositions of dielectric material are SiO2, MgO, Al2O3, and the value of thermal conductivity is much less than 20 W/m° K.
The supporting submount materials, such as FR-4, semiconductor material, pure metals, compound metal alloys, and compound ceramic materials, are commonly applied during the circuit fabrication process. As indicated in U.S. Pat. No. 6,885,035, semiconductor material was primarily used for the submount circuit. The thermal conductivity for typical semiconductor material is about 150 W/m° K. As shown in U.S. Pat. No. 6,455,930, LTCC-M was invented for heat sinking packages. Ceramic materials and a Cu—Mo—Cu related metal compound material were made for circuit boards. However, the complicated process and high production cost will limit the application of these materials.
In another example of the prior device, the ANOTHERM™ circuitry submount uses high temperature anodized 3003/6061 aluminum substrate, that grows up to 35 μm of oxide layer for electrical insulation. The substrate thermal conductivity is about 173 W/m° K and basically the circuit board is fabricated by using a screen printing method.
Another consideration in providing the submount for LED devices involves the morphology and mechanical properties of the submount. To form a stable and firm bond between the LED chip and the submount and between bonding wires and the submount, the morphology of the submount is preferably smooth, and the submount preferably is of sufficient thickness so that it can mechanically support bonds to the LED chip and bonding wires.